ABSTRACT Resistance to therapeutic agents is the predominant cause of high lethality of pancreatic cancer (PC). Inadequate and heterogeneous blood flow and obstructive desmoplastic stromal compartment are the major impediments to the delivery and intratumoral distribution of therapeutic agents and are the extrinsic determinants of therapy resistance in PC. Intrinsic chemoresistance of PC cells is dictated by a unique population of drug resistant cancer stem cells. Hence selective modulation of blood flow and extracellular matrix can lead to improved delivery of therapeutic agents into the tumors, while elimination of CSCs can improve the sensitivity of tumor cells to chemotherapy. Elevated endothelin (ET)-1 levels and overexpression of the two ET receptors (ETAR and ETBR) are observed in tumors. Importantly, ET-1 is a strong vasomodulator and induces vasoconstriction via ETAR is believed to be an important contributor to the tumor blood flow heterogeneity in tumors. Our preliminary studies indicate that the components of ET-axis are expressed in pancreatic tumors in various components of TME including tumor cells, blood vessel and stromal cells and CSCs. Further, targeting of ETAR with a specific inhibitor BQ123 selectively enhanced perfusion and reduced hypoxia in xenograft PC tumors, while prolonged inhibition of ET-axis in autochthonous tumors (in KPC mouse model of PC) with dual specificity inhibitor (Bosentan) resulted in marked inhibition of desmoplasia. We also observed that ET- 1 exerts pro-fibrogenic effects on murine pancreatic stellate cells via ETBR. We hypothesize that: ?Sequential inhibition of ETBR and ETAR can modulate stroma and perfusion for enhanced delivery, distribution and efficacy of therapeutic agents and clinically, combination of abraxane and gemcitabine will be more efficacious with the modulation of ET axis?. Three specific aims are proposed. Studies in Aim 1 will determine the effect of selective ETAR and ETBR antagonists BQ123 and BQ788, on the delivery and distribution of macromolecule (ABX) and small molecule-based (GEM) therapeutic agents. Biodistribution of radiolabeled ABX and GEM will be studied for quantitative estimation of tumor uptake. Further, we will decipher the mechanistic role of ET-axis-mediated cross-talk between cancer and stromal cells in PC in the context of desmoplasia. Studies proposed in Aim 2 will determine the impact of ET-axis targeting on the efficacy of ABX+GEM both in vitro (using unique cancer cell- stellate cell co-cultures and tumor organoid cultures) and in vivo using genetically engineered mouse (KPC) model of PC. Finally, Aim 3 is designed to clinically evaluate the safety and perform initial screening for efficacy of combining the ET-axis antagonist Bosentan with FDA-approved combination therapy Gemcitabine (GEM) plus Abrexane (ABX) in PC patients. Altogether, the proposed studies will demonstrate the preclinical feasibility of eliminating both extrinsic and intrinsic determinants of therapy resistance by targeting a single signaling axis and clinically determine the safety of combining ET-axis antagonism with approved chemotherapeutic regimens of PC. The data generated from the three aims will form the basis of future Phase 1/II clinical trials in lethal PC.